Solar Electric Propulsion (SEP)

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Solar Electric Propulsion (SEP)

"Following the light of the sun, we left the Old World."
-- Inscription on Columbus' ships

As NASA seeks to reduce costs and extend the length and capabilities of ambitious new solar system science and exploration missions, alternative propulsion technologies may deliver the right mix of savings, safety and superior propulsive power to enrich a variety of next-generation journeys to worlds and destinations beyond Earth orbit.

NASA’s Solar Electric Propulsion (SEP) project is developing critical technologies to enable cost-effective new trips to Mars and to asteroids across the inner solar system, and also to support a variety of commercial spaceflight activities. Energized by the electric power from on-board solar arrays, the electrically propelled system will use 10 times less propellant than a comparable, conventional chemical propulsion system -- such as those used to power the space shuttles to orbit. Yet that reduced fuel mass will deliver robust power capable of propelling robotic and crewed missions well beyond low-Earth orbit, sending exploration spacecraft to distant destinations or ferrying cargo to and from points of interest, laying the groundwork for new missions or resupplying those already under way.

NASA's Glenn Research Center in Cleveland, Ohio, leads the Solar Electric Propulsion project for the agency, and is preparing a system-level flight demonstration to launch later this decade.

During the technology maturation period -- under the auspices of NASA's Game Changing Development Program, prior to transitioning to the TDM Program -- the SEP project began developing large, flexible, radiation-resistant solar arrays that can be stowed into small, lightweight packages for launch and then unfurled to capture enough solar energy to provide high levels of electrical power. The project is working with ATK Aerospace and Deployable Space Systems, Inc. to build and test two solar arrays: one that folds out like a fan (ATK MegaFlex) and another that rolls out like a carpet (DSS Mega-ROSA). Both use lightweight structures and flexible blanket technology and are durable enough to operate for long periods in Earth orbit or passing through the punishing space environment, including the Van Allen radiation belts.

The Solar Electric Propulsion project also will use electrostatic Hall thrusters instead of a rocket engine with conventional chemical propellant. In Hall thrusters, the propellant is accelerated by an electric field. The thruster traps electrons in a magnetic field and uses them to ionize the onboard propellant -- in this case, the inert gas xenon -- into an exhaust plume of plasma that pushes the spacecraft forward. Such a system can be used to accelerate xenon ions to more than 65,000 mph.

The project is developing a durable Hall thruster with advanced magnetic shielding; several of these thrusters can be combined to increase the power of an SEP spacecraft. Also in development are new solutions for more efficiently converting energy gathered by the solar arrays into electricity. Thus, the thrusters will be able to operate at higher temperatures with reduced cooling needs, reducing weight, wear and overall cost of the system.

Later this decade, NASA will demonstrate a Solar Electric Propulsion system in flight, launching a test spacecraft to validate the technology and hardware during for a high-energy orbit-transfer mission typically conducted using chemical propulsion.

The Solar Electric Propulsion project is developing large solar arrays and high-power electric thrusters for an integrated in-space test-flight. Compared with current conventional chemical propulsion systems, at launch it will weigh two times less and use four times less storage for the electricity produced, and will operate at radiation levels four times greater than current commercial systems.

Because the SEP spacecraft will weigh much less at launch, fewer heavy launch vehicles will be needed to enable missions to Mars or near-Earth asteroids. Each launch vehicle will be able to carry more supplies or science instruments, potentially saving billions of dollars.

The system’s 30- to 50-kilowatt power level will significantly increase SEP capabilities, permit wider launch windows and open up a range of possible missions, including robotic missions to redirect an asteroid into lunar orbit for study; science missions to determine the effects of long-duration space missions on living cells and organisms; commercial use to service and reposition orbital communications satellites; and a variety of cost-effective robotic and crewed missions to Mars or other solar system destinations.